It has long been recognized that warming intravenous fluids to body temperature is beneficial and for rapid infusions of blood or other cold fluids such warming is necessary to prevent cardiac arrhythmias and possible cardiac arrest.
Fluid warmers have several challenges to meet. First, blood must not be overheated, or lysis of red cells occurs making the infusion toxic. Second, high flow rates are sometimes needed to replace blood volume in the event of rapid surgical blood loss. Most surgical cases, however, use only 1 to 2 liters of intravenous fluids over one or more hours, at low flow rates. Only a few blood warmers can meet the challenge of high flow rates (up to 500 ml/min), and most blood warmers can only effectively warm fluid at the low flow range down to about 25 ml/min.
The present invention provides both high and low flow rates (warms fluid from 10 to 35C from zero to 600 ml/min). Further, it provides this superior performance using a single, low cost disposable.
My previous U.S. Pat. No. 5,420,962 related to a disposable system that incorporated a hydrophobic vent patch into the disposable envelope heat exchanger. It also provided for preservation of heat in the patient i.v. line by passing the i.v. line through a larger diameter (about 1 inch dia) flexible corrugated plastic tubing. Warm air was passed through the outer tube, bathing the i.v. line and reducing the heat loss to the ambient air.
U.S. Pat. No. 5,875,282 employs a flat envelope heat exchanger carried into position by a rigid plastic cassette and warms the heating plates directly with resistance heaters, but is less effective at high and low flow rates.
U.S. Pat. No. 5,063,994 utilizes a patient line with a central intravenous fluid lumen surrounded by a warm water carrying annular lumen which is divided in half. The warm water flows toward the patient in one half of the annular lumen, turns around 180 degrees and returns to the blood warmer, actively warming the intravenous fluid, but is only effective at low flow rates.
The present invention provides superior low flow rate performance by conductively heating the outlet drip chamber using heat from one of the heating plates, and also by passively insulating the i.v. line to the patient by employing an annular air space which surrounds the i.v. line with still air. The patient line tubing is a single extrusion, about 0.37 inch outside diameter polyvinyl chloride or other flexible plastic. This passive insulation of the patient line allows delivery of 35C fluid 4ft from the warmer outlet as low as 15 ml/min. A further improvement of the present invention is the addition of an optionally employed external temperature-controlled sensorless or sensor-controlled warmer to the distal portion of the patient intravenous line, actively warming and insulating approximately the distal 2 ft of patient line. For pediatric or other extremely low flow uses, this active warming of the distal line allows delivery of 37C fluid essentially down to zero flow rate.
A new means of loading the flexible, floppy flattened envelope heat exchanger into the blood warmer apparatus is simple and economical, employing a paperboard inserter that passes easily through the very narrow slot between the two heating plates and then is grasped at the opposite end of the blood warmer and used to pull the envelope heat exchanger into place. This inserter eliminates the more complex stiff plastic cassette often used currently, and also allows closer spacing of the heating plates, resulting in more efficient heat transfer.
At low flow rates, blood cools by convection as it flows to the patient, negating the value of the blood warmer apparatus. The conductively warmed and externally insulated drip chamber holder of this invention in synergy and complementary action with a patient intravenous line that is insulated by an annular air space co-extruded with the central blood tube significantly reduce convective heat loss to the cooler ambient air. An optionally employed, temperature-controlled, reusable external heater may be applied to the distal portion of the patient line, allowing delivery of normothermic intravenous fluid to the patient at very low flow rates, down to essentially zero flow rate.
The improvements of this invention allow a single, low cost disposable system to provide warm blood or other fluid to the patient over the entire clinical range of useful flow rates, warming blood from 10 degrees C. to at least 35 C from zero ml/min to 600 ml/min by a compact, easy-to-use intravenous pole-mounted apparatus weighing less than 10 pounds.
Accordingly, several objects and advantages of my invention are as follows. The present invention employs a thin semi-rigid paperboard inserter to load the flattened plastic envelope disposable heat exchanger between the fixedly mounted parallel heating plates which are closely spaced apart. Use of this inserter in place of a rigid plastic cassette to carry the disposable envelope allows the heating plates to be closer together, resulting in higher efficiency heat transfer.
A further object of the present invention is to allow delivery of warm fluid at low flow rates to the patient. Advantages working together in synergy to produce this result are the use of a heat conductive drip chamber holder sleeve closely attached to one of the heating plates to keep the drip chamber warm, and a flexible plastic patient line i.v. tubing with larger outside tubing extruded as part of the patient line. An annular air gap between the outer and inner tubes passively insulates the i.v. line from the cooler ambient air. Also, an optionally employed reusable external sensorless or sensor-controlled electric heater applied to the distal portion of the patient line actively warms fluid just before delivery to the patient at very low flow rates. These three improvements acting together in synergistic complementary action provide improvements in low flow performance not achieved before.
Another object of the present invention is to automatically vent high flow rates of air or bubbles from the flow path before infusion of fluid into the patient. It is well-known that infusion of air or gas into the veins of a patient may be lethal. In the present invention, a large (about 1 inch in diameter) hydrophobic vent is incorporated into the top of the drip chamber, capable of venting large volumes of air quickly. A commercially available check valve prevents reverse flow of air into the fluid flow path.